1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED), and more particularly, to a driving circuit for driving the OLED by determining input data.
2. Description of the Prior Art
Having a variety of advantages, such as high light intensity, high response velocity, wide viewing angle, spontaneous light source and thin bulk, an organic light emitting diode (OLED) is becoming one of the most popular light emitting components that form a display device. Since an OLED is a current-driving component, the intensity (gray scale) of light emitted by an OLED can be controlled by determining levels of currents flowing therethrough.
A voltage-driving method, a method for driving an OLED by determining currents flowing therethrough, controls a voltage at a gate of a thin film transistor (TFT), which is connected to an OLED, and currents flowing through the OLED, and adjusts light intensity of the OLED. The larger the voltage difference between the gate and a source of the TFT is, the stronger the currents flowing through the OLED become, and so does the corresponding gray scale. On the contrary, if the voltage difference between the gate and the source of the TFT is becoming smaller, levels of the currents flowing through the OLED are becoming weaker and the light intensity of the OLED becomes fainter gradually, generating the gray scale of a small value.
Though a TFT fabricated in a low temperature poly-silicon process has an advantage of high carrier mobility, which promotes the performance of an OLED driven by the TFT dramatically, in the process of fabricating TFTs, two TFTs of the same type usually have two unequal threshold voltages, resulting in a problem of uneven image-displaying. That is, two same-typed TFTs can only generate two kinds of current levels even if these two TFTs are applied by two identical driving voltages respectively, resulting that two OLEDs driven by two same-typed TFTs applied by two identical driving voltages can only emit lights with two different levels of intensity instead of generating two gray scales of the same value, restricting the practicality of the OLEDs. However, currents generated by two same-typed TFTs operated on a saturation region differ slightly even if these two TFTs have two unequal threshold voltages, so driving TFTs, which control operations of OLEDs, into the saturation region enables these TFTs themselves to generate currents with equal levels despite that the threshold voltages of these TFTs differ from each other. In this scenario, levels of gray scale performed by an OLED can be adjusted by determining how long a period when currents flow through the OLED is.
A pulse width modulation (PWM) method is a method to control the intensity of light emitted by an OLED by providing a constant current to flow through the OLED and by controlling the period when the current is to flow through the OLED. Please refer to FIG. 1, which is a timing diagram of the PWM method according to the prior art. The PWM method divides a frame SF into N (N is equal to 6 in this example) subframes SF0 to SF5 according to a gray scale 2 N. The subframes SF0 to SF5 each comprise a data-writing period and a data-displaying period. For example, the subframe SF0 comprises a data-writing period TV0 and a data-displaying period TL0, and likewise do the subframes SF1 to SF5. A TFT that controls an OLED to emit light can be driven into a cutoff region or into the saturation region during data-writing periods of an identical length in every frame by a constant voltage according to digital input data, which are transformed from analog input data by an analog digital converter (not shown), and then generates a constant current that controls the OLED to emit light of equal intensity while being driven into the saturation region. Therefore, depending on operating on the cutoff or saturation region, the TFT controls the OLED not to emit light or to emit light for a period corresponding to the length of a data-writing period and controls the OLED to demonstrate gray scale of different values according to the input data.
For example, the levels of the gray scale are assumed to be equal to 64, and the frame SF is to be divided into six subframes SF0 to SF5 and a length ratio between the data-displaying period of the subframes SF0 to SF5 is 1:2:4:8:16:32. If what the OLED is to perform has a gray scale of 27, the constant voltage drives the TFT into the saturation region during the data-writing period TV0, TV1, TV3 and TV4 of the subframe SF0, SF1, SF3 and SF4 (27=1+2+8+16) and the OLED emits light during the data-displaying period TL0, TL1, TL3 and TL4. In another example, if what the OLED is to perform has a gray scale of 55, the constant voltage drives the TFT into the saturation region during the data-writing period TV0, TV1, TV2, TV4 and TV5 of the subframe SF0, SF1, SF2, SF4 and SF5 (55=1+2+4+16+32) and the OLED emits light during the data-displaying period TL0, TL 1, TL 2, TL4 and TL5. The PWM method generates a gray scale corresponding to input data by controlling the length of time for the OLED to emit light (27/55=(TL0+TL1+TL3+TL4)/(TL0+TL1+TL2+TL4+TL5)) and overcomes the drawback of unevenly-displaying image performed by an OLED driven by a TFT under the voltage-driving method.
However, in the process of controlling the OLED to emit light under the PWM method, no matter what the input data are, the OLED does not emit light during any data-writing period. That is, the light efficiency of the OLED can only be as high as a ratio between the total length of time of the data-displaying periods and the length of time of the frames ((TL0+TL1+TL2+TL3+TL4+TL5)/(SF0+SF1+SF2+SF3+SF4+SF5), therefore reducing the light efficiency of the OLED. Moreover, as the gray scale increases, the number of the subframes has to increase accordingly. Therefore, the length of time that each of the subframes can share with decreases. A subframe of a shorter period indicates a data-writing of a shorter period. An OLED driving circuit usually increases/decreases a voltage level of a capacitor by charging/discharging the capacitor and controls the intensity of light emitted by an OLED. Too short a data-writing period cannot provide the driving circuit enough time to charge/discharge the capacitor to a voltage exactly corresponding to the input data. In addition, as the working frequency of the OLED increases, the length of time of the display SF, as well as the data-writing period of each frame, decreases accordingly. Although a capacitor of a larger capacitance can be used to overcome the drawback, such a capacitor can only occupy large area, contradictory to the principles of light weight and small bulk that a modern integrated circuit demands. In such a scenario, the PWM method can only control an OLED to display an image of a gray scale of a limited numbers under a presumption that the capacitance of a capacitor cannot over a predetermined value. Another drawback of the prior art the PWM method is that it can only deal with digital input data. Therefore, the PWM method further needs an analog digital converter to convert analog input data into digital input data, increasing the cost of the driving circuit.